1. Field of the Invention
The present invention relates to an active matrix light emitting device. More specifically, the invention relates to a structure of a part from which light is extracted.
2. Description of the Related Art
A light emitting device utilizing light from an electroluminescent element (a light emitting element) attracts attention as a low power consumption display device having a wide viewing angle.
There are an active matrix type and a passive matrix type as driving methods for a light emitting device mainly used for display. In an active matrix light emitting device, an emission state, a non-emission state or the like of every light emitting element can be controlled. Therefore, an active matrix light emitting device can be driven with lower power consumption than that of a passive matrix light emitting device, and it is suitable for being mounted not only as a display portion of a small electric appliance such as a cellular phone but also as a display portion of a large-size television receiver or the like.
In an active matrix light emitting device, each light emitting element is provided with a transistor for controlling the drive of the respective light emitting element. The transistor and the light emitting element each are placed over a substrate so that the extraction of light toward the outside is not prevented by the transistor (for example, refer to Patent Document 1: Japanese Patent Application No. Hei08-330600 and Patent Document 2: Japanese Patent Application No. Hei10-254383). The light emitting element has a structure in which a light emitting layer is interposed between a pair of electrodes, and light is generated in the light emitting layer. Here, an emission state and a non-emission state of the light emitting element are controlled by a signal from the transistor.
Light is extracted toward the outside through a transparent electrode formed from indium tin oxide (ITO) and a substrate formed from quartz, like a light emitting device in Patent Documents 1 and 2. At this time, refractive indexes of ITO, quartz, and a substance which constitutes a light emitting layer are different with each other; therefore, a part of light is reflected in the interface between the light emitting layer and the transparent electrode and in the interface between the transparent electrode and the substrate. Due to such reflection of light, light extraction efficiency toward the outside of the light emitting device is reduced.
Moreover, in recent years, an active matrix light emitting device using a substrate formed from not quartz but glass or plastic has been developed to make a light emitting device lightweight or the like. However, a substrate formed from glass includes impurities such as an alkali metal and the like. In addition, when impurities of an alkali metal and the like are attached to a substrate formed from plastic, the substrate formed from plastic easily diffuses impurities. It is concerned that a transistor does not work normally if the impurities are mixed into the transistor. Therefore, an insulating layer for preventing the diffusion of impurities is provided between the substrate and the transistor. However, in the case where the insulating layer for preventing the diffusion of impurities is provided, it is concerned that light is reflected in the interface between the insulating layer and a transparent electrode or in the interface between the insulating layer and the substrate, and thus, light extraction efficiency is reduced as described above.